Disc brake calipers are well known in the art of vehicle braking systems. In such systems, a master cylinder generates a hydraulic brake signal that is communicated to several brake actuators via brake lines. The signal drives a piston disposed within the caliper, which in turn forces an inboard brake pad into frictional engagement with a rotatable brake disc or “rotor.” A reaction force is also generated, which causes the caliper to move inboard, drawing an outboard brake pad into frictional engagement with the rotor. Upon removal of the brake signal, the brake pads retract from the rotor to prevent frictional driving losses and undue wear on the braking components.
It is also known to utilize the brakes of a vehicle for both service and parking braking. Such brakes are typically actuated hydraulically when used for service braking. A separate mechanical actuation means is used to actuate the brakes for use as a parking brake. For example, a shaft or lever may be used to rotatably transmit a parking brake force to a clutch or cable that moves a piston to an extended, brake-engaging direction. Rotation of the lever in the opposite direction releases the clutch or cable and permits retraction of the piston.
Electrical actuation of a parking brake is also available in the art. In these systems an electric motor applies a force to a mechanical parking brake system by acting on either a drum or disc brake. Advantages of electrically-actuated parking brakes include ease of operation and consistent application of a predetermined amount of braking force. However, electrically-actuated parking brakes typically require large electric motor actuators that in turn require a relatively large amount of electrical power to actuate.
A particular problem with mechanical or electrical actuation of a hydraulic service brake for use as a parking brake occurs when a separate hydraulic pressure source is unable to supply adequate hydraulic pressure for applying and releasing the parking brake. Under such conditions, insufficient parking brake clamp force may be present to keep the vehicle from moving, particularly if the vehicle is parked on an incline.
There is a need for a hydraulic service brake that can provide sufficient clamp force for use as a parking brake under all vehicle operating conditions. There is a further need for a parking brake system that does not require a relatively large amount of electrical power to actuate.